As government regulations and consumer demands continue to motivate improvements in vehicle performance, more sophisticated control of vehicles and vehicle accessories is necessary. Real-time and near real-time control of vehicle systems and subsystems necessitates a reliable control system to gather and process a significant amount of information for generation and delivery of the appropriate control commands. For example, control of systems and subsystems such as the engine, transmission, anti-lock brakes, active suspension, and the like, requires relatively high-speed communication of information gathered from sensors to a processing unit. The processing unit analyzes the information and communicates the necessary control signals to various actuators to effect control of the system.
Various vehicle accessories such as power seats, power windows, power mirrors, an entertainment system, turn indicators, windshield wipers, climate control system, and the like, have one or more analogous control systems. Regardless of the complexity or the number of control systems, they all share common characteristics, such as a generalized control function which may be implemented by a relatively small number of generic building blocks. The control functions include gathering information, analyzing the information, and acting on the information to effect control of the system. The basic building blocks include sensors, actuators, and processing units.
Traditionally, the complexity and criticality (or lack thereof) of the various systems and accessories found in automotive applications required a number of dedicated processing units. The vehicle control system was essentially an aggregation of a number of stand-alone control systems with a minimum amount of inter-system communication. Systems and accessories were added to the vehicle in a piecemeal fashion as dictated by consumer demand and the capacity of existing cost-effective control systems. Often, the only common link between various control systems was the power provided by the vehicle battery or alternator.
The various sensors, actuators, and processors of traditional vehicle systems are connected by point-to-point conductors bundled in a vehicle wiring harness which also provides power distribution about the vehicle. As additional control systems and accessories are added, the wiring harness becomes more and more complex. This increased complexity often results in difficulty during the original assembly of the vehicle and in any subsequent diagnosis and repair of the vehicle electrical system. Furthermore, the addition of an "after-market" system or accessory to an existing vehicle is often an arduous task.
The continuing evolution of microelectronics and microprocessors is leading to more sophisticated and reliable processing units capable of integrating control of various vehicle systems and accessories. While such integration often improves the flexibility and coordination of control among various vehicle systems, it often requires the allocation of significant resources in developing customized electronic integrated circuits to address the needs of a particular application. Furthermore, most vehicles having integrated control systems still utilize a customized wiring harness having a number of conductors for point-to-point electrical communication among the various elements.
However, the prior art lacks a single set of electronic integrated circuit chips (microprocessors, microcomputers, input/output interfaces) which can accommodate various input and output signal types over the full range of complexity typically encountered in vehicular applications. As a result, there continues to be a proliferation of integrated circuits handling the input signals, information and control processing, and the output signals, with varying levels of integration. In addition, redundancy for critical control tasks, parallel processing, and other design parameters often lead to significantly more expensive implementations than desirable.
Recently, a number of approaches involving multiplexing of vehicle communications have been advanced to reduce the complexity of the wiring harness while integrating control of various vehicle systems. However, the hostile operating environment of a typical vehicle poses a number of problems in developing a reliable high-speed communication system. For such applications, a communication system must have sufficient reliability and bandwidth (or bit rate) to accommodate real-time control of systems which are essential to operator safety. The system must also be sufficiently immune to electromagnetic interference (EMI or noise) generated by the operation of various switches, motors and other electronic circuits. As is known, this typically requires some physical separation between power distribution lines and control signal delivery lines. Additionally, the system must be cost-effective to implement when compared to the relatively inexpensive, although cumbersome, traditional vehicle wiring harness.
In an effort to categorize various requirements of vehicle communication systems, the Society of Automotive Engineers (SAE) has divided automotive communication requirements into three classes denoted Class A, Class B, and Class C systems. Class C specifies the highest performance in terms of communication speed and accuracy while Class A refers to relatively low-speed signals which may be utilized for non-critical control of systems and accessories. A Class C communication system, having a data rate in excess of 125 kilobits per second (kbps) with a latency time of less than 5 milliseconds (mS) is necessary for real-time control of critical systems such as the powertrain or anti-lock braking system. Class B systems are often utilized for diagnostic functions, either on-line or off-line, and other information-sharing functions.
A number of communication system architectures or topologies may be utilized with a number of communication protocols to implement a multiplexed vehicular control system. A star topology has a central controller connected to various satellite controllers. This topology has the disadvantage that if the central controller malfunctions, the satellite controllers cannot communicate with one another. A linear bus topology accommodates expandability and configuration flexibility. This architecture allows controllers to communicate with one another or with less sophisticated modules while accommodating configuration modifications. Thus, the addition or elimination of a module does not significantly impact the physical or logical connections of the various modules communicating on the system.
A ring topology has a number of controllers connected in a ring arrangement. The communication network may pass through each controller or each controller may simply "attach" to the ring which acts as a communications bus. A ring topology using bus connections has built-in fault tolerance since if any single segment of the ring connection is broken, the ring structure degenerates to a linear bus structure so the various controllers can still communicate.
The various network architectures may utilize a number of communication protocols depending upon the particular application. For real-time control of critical systems, such as those encountered in vehicular applications, a deterministic communication protocol is desired. A deterministic protocol assures that critical information and control signals can be communicated within a predetermined period of time, i.e. a particular controller will not have to wait longer than that time to communicate when the network is busy with other communications. However, these protocols still have an associated finite delay time and so network control is not as fast as direct point-to-point control.
Communication protocols employing a prioritized deterministic multiple-access strategy are often used in vehicular applications. These protocols provide for non-destructive arbitration among communicating modules when contention for network access exists. For example, contention exists when two (2) or more modules request network access at the same time. An arbitration strategy determines which module will be allowed to communicate over the network. A prioritized non-destructive arbitration strategy, such as described by the SAE J1850 recommended practice, preserves the highest priority message so that it is communicated in a timely manner. Such a protocol accommodates sharing of communication resources among peer modules which control systems and accessories having disparate priorities, such as control of the vehicle engine and control of the vehicle entertainment system.